The engine starts and the blades can turn. Both of the belts are on, but the mower will not move in either direction. The blades were raised before going over a tree root. the mower rock back and forth because of the root and the mower stopped

Stop the engine, disconnect the spark plug wire(s).
Try moving the blades by hand.
Without being engaged, the blades should have a little resistance because they are still connected to each other. But shouldn't be hard to turn.
This will start you in the direction of finding out what's wrong.
A belt that's not in place, a bearing for one of the blades going bad, ect.

Even if you have slipped or broken the blade belt, that would have nothing at all to do with keeping it from starting UNLESS you are trying to start it with the blades still engaged. Check that.

Tighten or replace drive belt. Before you do that though, with the deck raised right up and engine switched off reach underneath deck and grab hold of blades and try to move them up and down. Make sure there is no build up of debris between pulleys and guards. Remove the belt off pulleys if you can. Turn blades and check they are free to turn. What you are doing here is checking the blade spindle bearings for wear or damage. If they have excessive movement or feel rough and/or tight replace them. If they are good and the belt looks in good order, tighten belt That is done by adjusting the deck rods so the deck moves back. You tube have some videos of belt adjustment.

blade brake is engaged unhook it and try turning the blade by hand should work

With most small lawn tractors I've dealt with, you want to unhook the deck-blade drive belt, some have one large belt that turns the blades as well as the rear wheels, others have more than one belt.. usually you will find four brackets that suspend the entire mower deck below the lawn tractor. To locate these brackets raise the mower deck so that its not resting on the ground, then push the mower deck side to side and notice where the attaching arms are moving under mower but above the deck, once you find the four brackets, lower the deck 2 its lowest point. remove the connecting pins noting the exact hole each pin came out of, if you have not removed the belt bye now do so at this time. you may find a stubborn piece of metal preventing the belt from coming off near the larger pulley Don't bend it out of the way just remove or loosen its nut,its there 4 a reason. so now, 1 the belts off the pulley. 2. the pins or bolts have been removed from the suspending arms and the hole positions have been noted. remove any other actuating rods or levers remaining so that the mower deck can slide out from under the mower. you may have to put the front wheels on a 4x4 blocks or similar to accomplish this. once you've got the deck out carefully turn it upside down, use blocks to keep from mashing pulleys, etc. you will probably see two or three blades, remove hex head bolt w/ washer holding blade to spindle, be sure to note cutting side of blade and direction of travel. If you don't see bolts in the center of blade look for my other solution for changing blades and blade bearings.

did you check blade belt and check to see if blade is bent if not you have a broke blade houseing

Is the belt in place that drives the blades?
Is the belt at the correct tension?
With the mower engine off.....try to turn each pulley the belt drives and on the mower deck with the mower in up position and or down....do they turn a little......this is to check for a seized bearing on one of the pulleys also look under the deck for something stuck........ jamming a blade.
Also check for the mower belt safety system (which stops the blades quickly when disengaging them) that may be stuck in stop mode.
With the mower engine running.....have some one engage the blades while you look under at the belt system....is the engine driving the belt.....is the belt driving the blades.
DO NOT TOUCH ANYTHING......MOVING OR STOPPED while the engine is trying to drive the blades.

on the engine there is a shaft with 2 pullys ,drive belt goes to the top closet to the engine lower pully is for the deck
there also is a saftey switch located on the deck which should shut down blade operation when raised. check the mandrels for broken bearings by grabbing the pully and wiggle them back and forth looking for movement could be possible that one madrel is shot giving you that bad sound which is crushed bearings

raise the front of your mower up and support it with something safe like a jack stand
take the blade off
when your blade is off locate the plastic cover covering the belt and remove
now remove the belt but first remove the plastic cover on top of the mower to uncover the gearbox that turns the wheels your belt goes from the engine pulley to the gearbox pulley
you will have to work the belt off the pulley while turning the motor by hand or the same when installing the belt.
to install belt reverse the procedure.
hope this helps some

Maintaining Riding Mowers Regular maintenance can reduce repairs.

Components of a typical riding lawn mower. (click to enlarge)

It's fun to ride around the yard on the mower, watching scrubby grass (and weeds) turn into trimmed lawn. However, when the grass looks as bad after you mow as it did before, or the mower doesn't want to start, riding mowers aren't as much fun. That's when it's time to put on your Fix-It Club hat and have some more fun.

A riding lawn mower is a gas-powered machine for cutting grass. The engine turns a rotating blade that cuts the top off the grass blades to a specified height. The operator sits atop the mower on a tractor-type seat with speed and height controls nearby. Riding mowers also can be used to pull small utility carts for other yard jobs.

To remove the mower deck:

1. Make sure that the parking brake is locked.
2. Place scrap lumber under the front and rear of the deck.
3. Use the deck lever to lower the deck.
4. Remove attachments from the deck.
5. Remove the blade drive belt from the front pulley.
6. Remove the front and rear deck fasteners, following instructions in your owner's manual.
7. Use the deck lever to raise the deck.
8. Remove the scrap lumber.
9. Remove the deck from under the mower.

To adjust the drive belt:

1. Remove the belt from the large idler pulley.
2. Remove the belt from the adjustable idler pulley.
3. Carefully remove the spring-loaded large idler pulley.
4. Loosen the adjustable idler pulley and move it slightly toward the rear of the mower to tighten it[md]or move it toward the front to loosen it—and retighten the pulley.
5. Replace the belt.
6. Check belt tension and readjust as needed.